Environmental Engineering Reference
In-Depth Information
mean horizontal stress to the vertical stress. It can be seen that horizontal stresses are gen-
erally significantly greater than vertical stresses, at depths of less than 500 m, and that at
greater depths the stresses tend to equalize.
2.5.1
Probable source of high horizontal stresses
The prime source of high horizontal stresses is believed to be tectonic forces, i.e. the forces
which drive and resist the motion of the earth's crustal plates. Evidence for this comes
mainly from depths of more than a kilometre below the surface, and is provided by analy-
ses of spalling (breakouts) and tensile fractures in the rock around deep wells drilled for
petroleum.
The horizontal stress fields inferred at these great depths have been related generally to
plate motion, and a world stress map has been produced (Zoback, 1992). There is also a
stress map covering Australia and Papua New Guinea (Hillis, 1998).
At the much shallower depths relevant to dam projects, the local stress fields often can-
not be related directly to the regional tectonic stress field at depth. The differences are
probably caused by local effects such as variations in topography or structure of the
bedrock.
It is possible also in some situations that the near-surface stresses result from strain
energy which has been locked into rocks during their formation, often, but not necessar-
ily, at great depth. This may have occurred in igneous rocks during their solidification,
and in sedimentary and metamorphic rocks, during compaction, cementation or recrystal-
lization; Emery (1964), Savage (1978), Nicholls (1980), Brown and Windsor (1990) and
Brady and Brown (1985). At a microscopic level the strain energy is considered to be
Figure 2.3.
Plot of vertical stresses against depth below ground surface (from Brown & Hoek, 1978, by
permission of Pergamon Press).
